There is often a need to monitor the flow of liquids or gases when the rate of flow is very low or when the need to detect a flow has added restrictions that limit the degree of interaction between the means for measuring the flow rate and the fluid itself. Typical applications of restrictive flow monitoring environments include chemical processing plants, where the fluids may be highly combustible or corrosive, or in facilities where there is a danger of electrical shock. A further example involves an X-ray laser, where there is a need to measure the flow of the cooling water through a high field pulsed magnet. Under typical conditions the rate of flow of water through the magnet is approximately 0.333 gallons per minute and the magnetic field coils are electrically pulsed to voltages exceeding several thousand volts; these conditions subject the flow monitor to very low rates of flow and a very high electric field.
There are several methods of monitoring fluid flow in a conduit or pipe. One of the methods of determining flow employs a magnetic switch pickup which uses a magnetic washer that is free to move over a short distance under the influence of a pressure differential across the washer. A magnetic reed switch is placed near the material, and when the fluid is flowing and the pressure differential is sufficient to cause the washer to move, the switch is engaged and closes indicating a fluid flow. This method allows for the measurement of flow in an opaque fluid but is usually restricted to high flow rates. Also, since the washer is imbedded in the flow, it provides a constriction to the path of flow and in addition, may prove to be position sensitive.
Another method of measuring flow, which is applicable to low flow rates, involves the use of a thermistor and a heating element. This method places a heating element upstream from a thermistor. The temperature detected by the thermistor is a function of the power supplied to the heating element, the specific heat of the fluid and the flow rate. Thus, knowing the power supplied to the heating element and the specific heat of the fluid, one can calibrate the fluid flow rate. One problem with this method is that both elements, the thermistor and the heating element, invade the flow stream. In addition, this method often encounters calibration problems.
Another method employs the doppler effect. With this method, a sound wave is transmitted into the fluid flow, and the frequency of the returning sound is detected. Movement of the fluid shifts the frequency of the returning sound by adding a component to the frequency. This is a very accurate method of determining flow and can measure very low flows, but it is much more costly than applicant's method or the other methods described. This method may also be susceptible to noise or mechanical vibration.
Applicant's invention involves an apparatus capable of detecting the flow in a fluid stream having a flow rate less than 0.333 gallons per minute. Further, with applicant's apparatus, there is only limited interference with the flow of the fluid stream. The apparatus for monitoring low flow is also capable with a minor modification to bracket a desired flow rate such that the apparatus will monitor a low rate of flow and also monitor if the fluid flow rate exceeds a predetermined upper limit.
Accordingly, it is an object of this invention to provide a means of measuring the existence of a fluid stream having a low volume rate of flow.
It is still a further object of this invention to provide a low flow meter capable of having a variable low flow trip point so that the user can select the low flow "trip" level at which flow will be detected.
It is still a further object of this invention to provide an apparatus with limited interference with the fluid stream.
It is a further object of this invention to provide for modified circuitry which will monitor fluid flow within a set of high-low fluid flow rate conditions.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentalities and combinations particularly pointed out in the appended claims.